It is well-known that airborne microorganisms like germs, viruses, fungi and bacteria may cause diseases. Recent studies have indicated that indoor air pollution may pose a great health risk. Indoor air pollution is in most cases caused by the presence of unwanted particulates, unwanted chemical substances and/or unwanted microorganisms.
Unwanted particulates and unwanted chemical substances can in most cases be avoided by conventional techniques such as filtration and ventilation. The contamination by microorganisms creates a more serious obstacle.
HEPA (High Efficiency Particulate Arresting filters) can be used to filter the incoming air by retaining unwanted particulates and unwanted microorganisms such as bacteria.
However, the conditions of the filter medium, such as temperature and humidity, are very favorable to cause a proliferation and dissemination of the germs over the entire thickness and the entire surface of the filter medium. Already after a few hours of operating the filter is contaminated with microorganisms.
A contaminated central air system can become a breeding ground for biological contaminants and the air forced through the air system can distribute the contaminants throughout the building.
In order to avoid a proliferation and dissemination of germs, the filter medium has to be disinfected at regular times. This can for example be achieved by a chemical or thermal treatment.
At present also a photocatalytic process using ultraviolet radiation with titanium dioxide as photocatalyst is known to disinfect a filter medium. However, this technique is only suitable for surface filtration.